Processes for treatment of polluted water normally involve a screening step, in which solids are removed by passage of the water through a screen. The removed solids, the screenings, form a wet, water-containing sludge that has to be dewatered and in many cases washed before the it can be deposited, incinerated or taken care of in another way. In order to obtain a manageable sludge that can easily be transported, deposited or incinerated it is essential that the solids content of the dewatered screenings is as high as possible. The same type of dewatering, and/or solids/liquid separation, and the same type of problems as those described below are also encountered in many industrial processes e.g. the dewatering and washing of cellulose pulp in the pulp and paper industry and in the food industry when pressing fruit juices and/or extracting oils from vegetable or animal material, but for brevity the description below will use screenings from treatment of polluted water as an example.
Sludge of screenings from municipal wastewater treatment plants normally contains cellulose fibres. Such sludge is suited for dewatering in screw presses and/or in piston presses also called ram presses. These types of presses as well as wash presses based on these press types are collectively referred to as press or presses in the following text. In the screw press the sludge is introduced into a hollow cylinder, having a water pervious wall, in which a rotating press screw compresses the sludge so that water is pressed out of the sludge and can escape through the water pervious cylinder wall. In the piston or ram press the sludge is likewise introduced into a hollow cylinder, having a water pervious wall, but here the dewatering is achieved by use of a piston that is pressed into the hollow cylinder and thus pressing water out of the sludge. The water escapes through the water pervious cylinder wall. Both screw presses and piston or ram presses are normally provided with a compaction device for the sludge, where the sludge is compacted and further water separation takes place. Such a compaction device is usually an extension of the hollow cylinder and may have either solid walls or water pervious walls and may be straight or bent.
In a so called wash press wash water is introduced into the sludge, mixed with the sludge and then removed by pressing, carrying with it fine solids that are sent back to the treatment plant for treatment together with the polluted water. This procedure can be repeated until the desired cleanness of the sludge has been obtained. In that case for each repeated wash cycle the pressure on the sludge is released, wash water is introduced and mixed with the sludge and then removed by renewed pressing According to one wash procedure the sludge is initially dewatered by pressing after which the pressure on the sludge is released, wash water is introduced and mixed with the sludge and then removed by renewed pressing. Also this procedure can be repeated until the desired cleanness of the sludge has been obtained A wash press may be either of the screw press type or the piston press type.
A conveyor tube is normally connected directly to the press for transporting the dewatered screenings to a container or deposit. Such a conveyor may be arranged to transport the dewatered screenings horizontally or at any angle to the horizontal. When dewatering sludge screenings it is desirable and of great economic value to get the sludge screenings as dry as possible after dewatering and washing/dewatering. However, current methods for obtaining a dry sludge after dewatering lead to problems in the press/conveyor system caused mainly by problems in the transportation of the sludge in the tube conveyors that are normally used in conjunction with the presses. Since dry sludge is in the form of lumps, aggregates or a hard “sausage” it causes a high friction against the conveyor tube wall, so that it might get stuck in the conveyor and cause blocking of the same. This means that to enable transport of the dewatered sludge the desired distance and at the desired angle to the horizontal, the sludge has to be kept wetter than is desirable from an economic point of view.
The problem of combining dewatering the sludge to a high dryness in presses and transporting the dewatered sludge in a tube conveyor or tube screw conveyor is well known in the industry and a number of solutions have been tried but none of the known solutions has given a satisfactory solution to the problem.
It is, for instance, well known to use conveyor tubes with widening diameters in the direction of transportation to avoid blocking by dry sludge. This will work as long as the sludge and dewatering parameters are in accordance with the design values. However it is in the nature of sludge and sludge dewatering that the properties of the sludge are variable and this will directly affect the dewatering performance of the press. Therefore, when using conveyors with widening tubes, i.e. conical tube sections, one always has to allow for these variations by setting the parameters of the dewatering press so that it will produce a wetter sludge than would be possible in order to ensure that the conveyor does not block up as the sludge load and/or the sludge characteristics in the press changes. In order to be able to operate optimally for different sludge characteristics one would need a set of conveyor tubes with different slopes of the wall of the conical section. These tube sections would have to be changed in accordance with sludge properties, which would be impractical. The solution using widening conveyor tubes will consequently produce sub-optimal results in respect of dryness of the sludge and it will also be susceptible to disturbances if an unforeseen change in the parameters should occur. In order to be on the safe side for the conveyor the dewatered sludge is normally kept much wetter than could be achieved by the press.
Another way that has been tried to solve the problem at hand is to use a tube conveyor of a type that allows variation of the slope of the conical section. In this way a certain adjustment of the tube can be carried out during operation in case the sludge characteristic and dryness after dewatering should change, but this requires constant supervision or a complicated control system. Changes cannot be made quickly enough to properly adjust to changes in load or sludge characteristics and the span of possible settings of the slope is limited. A further drawback of this system is that it can only be used for transporting the sludge a short distance and it cannot be used for lifting the sludge more than a short distance to a conveyor and/or a container. The sludge will exit from this device in the form of big lumps, which requires a conveyor screw with a big diameter for the further transport of the sludge. The reloading into a big diameter screw conveyor is costly and impractical.
Sludge dewatering by use of screw presses, piston presses and wash presses is an important operation in wastewater treatment and industrial processes. The laws and rules guiding the disposal of sludge and waste products are getting stricter all the time and there is a great need for simple methods and devices by which dried sludge that is transportable over long distances and at desired angles to the horizontal can be produced. It is a hygienic requirement that the sludge should be dewatered and transported in closed equipment and conveyors. The lay-out of wastewater treatment plants often requires closed transport of the sludge over long distances and also vertically between floors due to building constraints.